1. Field of the Invention
The present invention relates to the technical field of wireless communications and, more particularly, to a hybrid equalization system.
2. Description of Related Art
A wireless transmitting signal may be interfered by noises, and also subjected to the problem of channel fading or multi-channel interference. In addition, inter-symbol interferences (ISIs) are present at a receiver. Accordingly, equalizers are used to overcome or reduce the ISIs.
Typically, the equalizers are categorized into several types: (1) an equalizer implemented with maximum likelihood sequence estimation, which is deemed as the optimal in view of error probability; (2) a linear filter with adjustable coefficients; (3) a decision feedback equalizer for cancelling the ISIs by means of a previously detected symbol.
FIG. 1 shows a configuration of a typical decision feedback equalizer (DFE). As shown in FIG. 1, the equalizer 100 includes a feed-forward equalizer 110, a feedback equalizer 120, and a decision device 130. The feed-forward equalizer 110 and the feedback equalizer 120 are each a symbol spaced equalizer. The decision device 130 is based on a previously detected symbol to generate a decisive value. The feedback equalizer 120 receives an output of the decision device 130 for use as an input to cancel a part of the ISIs in a currently estimated symbol.
FIG. 2 is a configuration of a typical predictive DFE. As shown in FIG. 2, the equalizer 200 includes a feed-forward equalizer 210 and a feedback equalizer 220. A symbol data r(n) is input to the feed-forward equalizer 210, and an adder adds output values of the feed-forward equalizer 210 and the feedback equalizer 220 to thereby obtain an equalized result y(n). The decision device 240 is based on the equalized result y(n) to generate a quantized symbol signal d(n) to indicate a coarse result for a wireless transmitting signal received. The quantized symbol signal d(n) is then fed back to the feedback equalizer 220. The decision device 240 can be a slicer to slice an output of the equalizers into special quantity levels. The slicing means that a continuous value is changed into discontinuously special orders of value. The comparator 250 compares the quantized symbol signal d(n) with the equalized result y(n) to thereby generate an error signal e(n). Basically, the error signal e(n) is a difference between the quantized symbol signal d(n) and the equalized result y(n). The error signal e(n) is sent to the feed-forward equalizer 210 and the feedback equalizer 220 in order to update their coefficients by a least mean square (LMS) operation.
When the receiver starts to receive a wireless signal, the transmitter sends a preamble to set the receiver or initialize the coefficients of the feed-forward and the feedback equalizers 210 and 220, followed by transmitting data by the transmitter. Thus, the feed-forward equalizer 210 and the feedback equalizer 220 at the receiver can effectively cancel the ISIs. However, in new wireless communication protocols, the amount of preamble data is typically set very little for increasing the amount of data transmission, and in this case the equalizer 200 cannot cancel the ISIs effectively by obtaining the coefficients of the feed-forward and the feedback equalizers 210 and 220. In addition, the transmission in a wireless channel is typically interfered by noises, as well as channel fading or multi-channel interference. Accordingly, the channel estimation cannot be effectively performed to update the coefficients of the feed-forward and the feedback equalizers 210 and 220.
Therefore, it is desirable to provide an improved hybrid equalization system to mitigate and/or obviate the aforementioned problems.